An optical distance detecting device with image sensors has been disclosed, for instance, by Japanese Patent Examined Publication Nos. Sho-63-38085/(1988) and Sho-63-46363/(1988). The device, as shown in FIG. 11, comprises two right and left optical systems which are made up of lenses 1 and 2 which are spaced by a base length L from each other.
Image sensors 3 and 4 are arranged at the focal length f of the lenses 1 and 2, respectively. The image signals of the image sensors 3 and 4 are applied to a signal processing unit 30, where they are successively shifted so as to be electrically superposed on each other. When the image signals are superposed best, the amount of shift (n) is detected to calculate the distance (R) to the object 31 by the following Equation (1) according to the trigonometrical measurement: EQU R=(f.times.L)/n (1)
On the other hand, a method of tracking the image of the car ahead which is picked up by an image sensor or the like has been disclosed, for instance, by Japanese Patent Examined Publication No. Sho-60-33352/(1985). In the method, a target selected is tracked as follows: The operator, while watching the display screen, sets a window on it in such a manner as to surround a target to be tracked while watching it.
In the conventional distance detecting device, the images picked up by the right and left optical systems are subjected to comparison to obtain the distance to the object. Hence, the following difficulty is involved when the device is mounted on a car to measure the distance to a car ahead. That is, if, during the measurement, other cars come on the right and/or left sides of the car ahead, then the operator (or the driver) cannot determine which of the distances the device has measured.
Let us consider the case where an image pickup device is mounted on a first car, and the distance from a second car ahead of it is measured with the device. If, in this case, the first car bounds because the road is uneven, then the window tracking the image of the second car ahead bounds with respect to the latter. On the other hand, when the second car ahead increases the speed, or the first car decreases the speed, then the distance between the two cars becomes longer. In this case, the ratio of the image of the second car ahead picked up by the image sensor to the window in area is decreased, so that it is liable to be affected by the image of the background. As a result, the image of the second car ahead is shifted from the window. Thus it is impossible to stably track the image of the second car ahead.